Usually, in order to prevent the lock nut from being released, a washer or a coil spring is inserted between the bolt and the nut. Conventional lock nut having a spiral coil spring between the bolt and the nut is disclosed in Korean patent No. 10-701782 and 10-701643. This lock nut includes a thread part on the inner surface of the upstream side of the fastening direction which is engaged with the bolt, and a non-thread part having a larger inner diameter than that of the thread part on the downstream side of the fastening direction and a spiral coil spring being touched on the inner wall of the non-thread part with an interval. In the non-thread part of the lock nut are formed an upstream step and a downstream step having a reduced diameter respectively to prevent the spiral coil spring in the non-thread part from leaving out.
On the surface of the downstream step of the lock nut facing the axial line of the bolt is formed a recess, in which the downstream bent portion extending outward at the downstream end of the coil spring is inserted and fixed, and the upstream end of the spiral coil spring is touched internally on the non-thread part, and is separated along the axial direction at the upstream step. And the spiral coil spring has the inner surface being clamped with the thread part of the bolt.
The above bolt is operated as follows. If a user inserts the bolt into the lock nut and turns it to the fastening direction, firstly the thread part of the bolt is engaged with the thread part of the nut, then the thread part of the bolt and the spiral coil spring is engaged to each other. Accordingly, the spiral coil spring is twisted forward fastening direction, and as the downstream bent portion is fixed in the recess, the spiral coil spring will be enlarged radially, then the front end of the thread part of the bolt would smoothly pass through the spiral coil spring. If the turning of bolt toward the fastening direction is stopped, the enlarged spiral coil spring is reduced radially to be fastened with the thread part of the bolt elastically. In this state, the spiral coil spring keeps fastening the thread part of the bolt elastically, so that the lock nut would not be released from the bolt by any unwanted external force.
But in case that the bolt is turned toward the release or unlocking direction for maintenance, as the downstream bent portion of the spiral coil spring is fixed in the recess, the spiral coil spring will be reduced radially, so the turning action of the nut toward the release direction becomes difficult. If the user turns the lock nut toward the release direction with high strength, the spiral coil spring would be broken down at the downstream bent portion, and the spiral coil spring without the downstream bent portion would remain in the non-thread part in spite of the turning action with high strength. As a result, the lock nut could not be released without damaging the lock nut itself.
In order to overcome the above problem, a solution may be proposed that an upstream bent portion extending radially outward is formed at the upstream end of the spiral coil spring similarly as that of the downstream end, and a slot extending radially is formed on the non-thread part facing the upstream bent portion. If the lock nut is turned toward the fastening direction as the upstream bent portion being in the slot, as the downstream bent portion of the spiral coil spring is fixed in the recess, the overall spiral coil spring would be radially reduced, and the upstream bent portion becomes deformed in the release direction along the inner surface of the non-thread part, then it will be engaged into the slot, so that the overall spiral coil spring will be radially extended. But this proposal has a problem that when the upstream bent portion is deformed along the inner surface of the non-thread part and is engaged into the slot, both ends of the spiral coil spring are fixed in the upstream bent portion and the downstream bent portion, so that, even though the upstream bent portion may be radially extended, the downstream bent portion still remains radially reduced, and it is yet difficult to release the lock nut from the bolt.
To solve this problem, another solution may be proposed that the diameter of the spiral coil spring may be enlarged only on the upstream end than the other part, so that when the lock nut is turned toward the release direction, only the upstream end of the spiral coil spring is reduced radially to prevent the thread part of the bolt from being fastened. But this solution also has a problem that, as the diameter of the upstream end of the spiral coil spring is larger than that of the other part, the other part of the spiral coil spring is separated more inwardly from the non-thread part than that of the conventional one, the size of the lock nut would be inevitably larger, which will result in the increased material cost and manufacturing cost. If the thickness of the spiral coil spring is reduced to cope with this problem, then the fastening force of the spiral coil spring against the thread part of the bolt would be reduced, so the reliable anti-release function would not be achieved.
And, if it is repeated that the spiral coil spring is enlarged and then reduced radially when the lock nut is fastened, and the spiral coil spring is enlarged and then reduced radially when the lock nut is released, the safety accident may happen when the lock nut is applied on the fastening device of the bridge, railroad, and so on.